Semiconductor memory device for controlling write recovery time

ABSTRACT

A semiconductor memory device includes a CAS latency mode detecting means for outputting a CAS latency control signal in response to a CAS latency mode; and an auto-precharge control means for controlling timing of an auto-precharge operation in response to the CAS latency control signal.

FIELD OF INVENTION

The present invention relates to a semiconductor memory device; and,more particularly, to a semiconductor memory device having ability ofcontrolling timing of an auto-precharge operation depending on a columnaddress strobe (CAS) latency mode.

DESCRIPTION OF PRIOR ART

An operational speed of a semiconductor memory device is an importantfactor to evaluate performance of the semiconductor memory device.Particularly, a write recovery time is one of factors which haveinfluence on the operational speed of the semiconductor memory device.

After a data is stored into a unit cell included in the semiconductormemory device, a precharge operation is performed. Herein, there existsa time period before performing the precharge operation for the data tobe stably stored into the unit cell not being prevented by the prechargeoperation.

The time period is called the write recovery time. That is, the writerecovery time is a minimum time required for stably storing the datainto the unit cell not being prevented by the precharge operation.

There have been introduced various ways for controlling the writerecovery time. For instance, the write recovery time can be controlledusing a clock signal. That is, in case that a mode register set (MRS) isset for a burst length to be 2, the write recovery time is a durationfrom a rising edge of the clock signal which a second data is inputtedsynchronizing with to a rising edge of the clock signal which aprecharge command signal is inputted synchronizing with.

Generally, there are three ways for controlling the write recovery time,i.e., a synchronous method, an asynchronous method and admixed method.

In case of the synchronous method, the write recovery time is controlledusing the clock signal as described above. That is, the prechargeoperation is performed synchronizing with the clock signal after theburst length and predetermined cycles of the clock signal. In case ofthe asynchronous method, the precharge operation is performed after theburst length and a predetermined delay time. The mixed method uses bothof the synchronous method and the asynchronous method.

Those methods for controlling the write recover time are selecteddepending on various factors such as an operating frequency, amanufacturing process, an operating voltage and a temperature of thesemiconductor memory device.

Generally, the synchronous method is adopted if there exist manyvariations on the manufacturing process; and the asynchronous method isadopted if the operating frequency is broad-banded.

FIG. 1 is a block diagram showing a conventional semiconductor memorydevice.

As shown, the conventional semiconductor memory device includes anauto-precharge control unit 10, a control unit 20 and a memory cellblock 30.

The auto-precharge control unit 10 generates a precharge control signalpc to control the control unit 20. The control unit 20 controls thememory cell block 30 in response to the precharge control signal pc sothat the memory cell block 30 performs an auto-precharge operation and adata access operation. The memory cell block 30 is provided with aplurality of unit cells.

To perform the data access operation, the conventional semiconductormemory device amplifies a data included in a selected unit cell, and theamplified data is latched by a sense-amplifier.

Thereafter, in case that the data access operation is performed forreading data from the conventional semiconductor memory device, the datalatched by the sense-amplifier is outputted to the outside of theconventional semiconductor memory device. In case that the data accessoperation is performed for writing data to the conventionalsemiconductor memory device, an inputted data to be written is latchedby the sense-amplifier. Herein, if the inputted data to be written isequal to the data latched by the sense-amplifier, the sense-amplifierkeeps its data. On the other hand, if the inputted data to be written isdifferent from the data latched by the sense-amplifier, thesense-amplifier inverts its data. Then, the data latched by thesense-amplifier is stored into the selected unit cell.

Thereafter, the sense-amplifier is precharged. This operation, i.e., theprecharge operation was performed by an external precharge commandsignal when the operational speed of a semiconductor memory device wasslow. However, at present, as a semiconductor memory device operates ata high speed, an internal prechrage command signal is generated and theprecharge operation is performed automatically. That is, aftercompleting an operation corresponding to an inputted command signal, theprecharge operation is performed automatically after a predeterminedtime is passed. This automatically performed precharge operation iscalled an auto-precharge operation.

FIG. 2 is a block diagram showing the auto-precharge control unit 10.

As shown, the auto-precharge control unit 10 includes a timing controlunit 11, an auto-precharge timing decoder 12 and an auto-prechargetiming control unit 13.

The timing control unit 11 generates a first auto-precharge controlsignal apcg1 and a second auto-precharge control signal apcg2 to controltiming of the auto-precharge operation after comparing timing of theauto-precharge operation expected when the conventional semiconductormemory device is designed with timing of the auto-precharge operationwhen the conventional semiconductor memory device is manufactured. Theauto-precharge timing decoder 12 decodes the first and the secondauto-precharge control signals apcg1 and apcg2 in order to output aplurality of decoded signals A, B and C. The auto-precharge timingcontrol unit 13 receives an auto-precharge performing signal apcgpb foroutputting the auto-precharge performing signal apcgpb as the prechargesignal pc after adjusting output timing of the auto-precharge performingsignal apcgpb in response to the plurality of decoded signals A, B andC.

Herein, as described above, the first auto-precharge control signalapcg1 and the second auto-precharge control signal apcg2 are used forcompensating time difference between timing of the auto-prechargeoperation expected when the conventional semiconductor memory device isdesigned with timing of the auto-precharge operation when theconventional semiconductor memory device is manufactured. The firstauto-precharge control signal apcg1 and the second auto-prechargecontrol signal apcg2 are also used for arbitrarily control timing of theauto-precharge operation.

FIG. 3 is a schematic circuit diagram showing the timing control unit11.

As shown, the timing control unit 11 includes a first and a second unittiming control units 11A and 11B.

The first unit timing control unit 11A includes a first fuse F1, a firstcapacitor C1, a first metal oxide semiconductor (MOS) transistor MN1 anda plurality of inverters I1, I2 and I3.

One side of the first fuse F1 is connected to a power supply voltage VDDand the other side is connected to the first capacitor C1, the first MOStransistor MN1 and the inverter I1. A gate of the MOS transistor MN1 isconnected to an output of the inverter I1. One side of the firstcapacitor C1 is connected to the first fuse F1, the MOS transistor MN1and the inverter I1 and the other side is connected to a ground voltageVSS. The inverters I2 and I3 serve to buffer an output signal from theinverter I1 for outputting the first auto-precharge control signalapcg1.

The second unit timing control unit 11B includes a second fuse F2, asecond capacitor C2, a second MOS transistor MN2 and a plurality ofinverters I4, I5 and I6.

The timing control unit 11 controls the first and the second prechargecontrol signals apcg1 and apcg2 by selectively blowing the first and thesecond fuse F1 and F2.

For instance, at a wafer-level test, if the first fuse F1 is blown by alaser, the first precharge control signal apcg1 is activated as a logichigh level because charge stored in the first capacitor C1 is dischargedand a voltage level of an input of the inverter I1 becomes a voltagelevel of the ground voltage VDD. In case that the first fuse F1 is notblown, the first precharge control signal apcg1 is outputted as a logiclow level.

Herein, the timing control unit 11 includes two unit timing controlunits, i.e., the first and the second unit timing control units 11A and11B. However, the timing control unit 11 can includes more unit timingcontrol units.

FIG. 4 is a schematic circuit diagram showing the auto-precharge timingdecoder 12 shown in FIG. 2.

As shown, the auto-precharge timing decoder 12 includes two inverters I7and I8 and three NAND gates ND1, ND2 and ND3.

The auto-precharge timing decoder 12 decodes the first and the secondprecharge control signals apcg1 and apcg2 for selectively activating andoutputting the plurality of decoded signals A, B and C.

Thereafter, the auto-precharge timing control unit 13 delays theauto-precharge performing signal apcgpb for a predetermined time andoutputs the delayed auto-precharge performing signal apcgpb in responseto the plurality of decoded signals A, B and C.

Meanwhile, timing of the auto-precharge operation is determined by thewrite recovery time as described above. The write recovery time isvariable depending on a column address strobe (CAS) latency. The CASlatency (CL) is the ratio between column access time and a clock cycletime. That is, the CL shows how many cycles of the clock signal arespent while performing a read operation of a semiconductor memorydevice.

However, if the write recovery time keeps its value regardless of theCL, a semiconductor memory device can operate abnormally when the CLchanges.

Generally, a semiconductor memory device changes value of the CLdepending on a frequency change of the clock signal to be operatedstably. The conventional semiconductor memory device can not control theauto-precharge operation depending on the CL. Therefore, there occursome errors when a value of the CL is changed due to the frequencychange of the clock signal.

SUMMARY OF INVENTION

It is, therefore, an object of the present invention to provide asemiconductor memory device having ability of controlling a writerecovery time depending on a frequency of a clock signal.

In accordance with an aspect of the present invention, there is provideda semiconductor memory device including a CAS latency mode detectingmeans for outputting a CAS latency control signal in response to a CASlatency mode; and an auto-precharge control means for controlling timingof an auto-precharge operation in response to the CAS latency controlsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of preferred embodimentstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing a conventional semiconductor memorydevice;

FIG. 2 is a block diagram showing an auto precharge control unit shownin FIG. 1;

FIG. 3 is a schematic circuit diagram showing a timing control unitshown in FIG. 2;

FIG. 4 is a schematic circuit diagram showing an auto-precharge timingdecoder shown in FIG. 2;

FIG. 5 is a block diagram showing a semiconductor memory device inaccordance with a preferred embodiment of the present invention;

FIG. 6 is a schematic circuit diagram showing a column address strobe(CAS) latency mode detecting unit shown in FIG. 5;

FIG. 7 is a schematic circuit diagram showing an auto-precharge timingdecoder shown in FIG. 5; and

FIG. 8 is a schematic circuit diagram showing an auto-precharge timingcontrol unit shown in FIG. 5.

DETAILED DESCRIPTION OF INVENTION

Hereinafter, a semiconductor memory device in accordance with thepresent invention will be described in detail referring to theaccompanying drawings.

FIG. 5 is a block diagram showing a semiconductor memory device inaccordance with a preferred embodiment of the present invention.

As shown, the semiconductor memory device includes an auto-prechargecontrol unit 1000 and a column address strobe (CAS) latency modedetecting unit 100.

The CAS latency mode detecting unit 100 generates a first and a secondCAS latency control signals CL_S₁ and CL_S2 in response to a CAS latencymode. The auto-prechrage control unit 1000 controls timing of anauto-precharge operation in response to the first and the second CASlatency control signals CL_S₁ and CL_S2.

The auto-precharge control unit 1000 includes a timing control unit 200,an auto-precharge timing decoder 300 and an auto-precharge timingcontrol unit 400.

The timing control unit 11 generates a first and a second prechargecontrol signals apcg1 and apcg2. The auto-precharge timing decoder 300decodes the first and the second precharge control signals apcg1 andapcg2 and the first and the second CAS latency control signals CL_SL andCL_S2 for generating and outputting a plurality of decoded signals A, B,C and D. Herein, the auto-precharge timing decoder 300 activates one ofthe plurality of decoded signals A, B, C and D.

The auto-precharge timing control unit 400 delays an auto-prechargeperforming signal apcgpb in response to the plurality of decoded signalsA, B, C and D for outputting the delayed auto-precharge performingsignal apcgpb. Herein, how long the auto-precharge performing signalapcgpb is delayed is determined by the plurality of decoded signals A,B, C and D.

The timing control unit 11 includes a plurality of fuses for outputtingthe first and the second precharge control signals apcg1 and apcg2. Thestructure of the timing control unit 11 is the same as that of thetiming control unit included in the conventional semiconductor memorydevice.

FIG. 6 is a schematic circuit diagram showing the CAS latency modedetecting-unit 100 shown in FIG. 5.

As shown, the CAS latency mode detecting unit 100 includes a first unitCAS latency mode detector 110 and a second unit CAS latency modedetector 120.

In detail, each of the first and the second unit CAS latency modedetector 110 and 120 receives two CAS latency mode signals forgenerating a CAS latency control signal. That is, the second unit CASlatency mode detector 120 receives a first and a second CAS latency modesignals CL2 and CL3 to output the first CAS latency control signalCL_S1. Likewise, the first unit CAS latency mode detector 110 receives athird and a fourth CAS latency mode signals CL4 and CL5.

The first CAS latency mode detector 110 includes two inverters I11 andI12 and a NAND gate ND5. The two inverters I11 and I12 receive the thirdand the fourth CAS latency mode signals CL4 and CL5 respectively. TheNAND gate ND5 performs a logic operation to outputted signals from thetwo inverters I11 and I12 to output the second CAS latency controlsignal CL_S2.

The structure and the operation of the second CAS latency mode detector120 are the same as those of the first CAS latency mode detector 110.

If a CAS latency is 2 or 3, the CAS latency mode detecting unit 100activates and outputs the first CAS latency control signal CL_S1. On theother hand, if the CAS latency is 4 or 5, the CAS latency mode detectingunit 100 activates and outputs the second CAS latency control signalCL_S2.

Herein, the CAS latency mode detecting unit 100 includes two unit CASlatency mode detectors. However, the CAS latency mode detecting unit 100can include more unit CAS latency mode detectors if there are more CASlatency mode signals besides the first to the fourth CAS latency modesignals CL2 to CL5. In addition, it is also possible to arrange the unitCAS latency mode signals differently.

FIG. 7 is a schematic circuit diagram showing the auto-precharge timingdecoder 300 shown in FIG. 5.

As shown, the auto-precharge timing decoder 300 includes an internaldecoder 310 and a signal mixing unit 320.

The internal decoder 310 including a plurality of inverters and NANDgates to receive and decode the first and the second precharge controlsignals apcg1 and apcg2; and, thus, to output the decoded signals to thesignal mixing unit 320.

The signal mixing unit 320 also includes a plurality of NAND gates andinverters. The signal mixing unit 320 receives the first and the secondCAS latency control signals CL_SL and CL_S2, and also receives thedecoded signals outputted from the internal decoder 310. The signalmixing unit 320 performs a logic operation on the decoded signals fromthe internal decoder 310 and the first and the second CAS latencycontrol signals CL_S₁ and CL_S2 in order to output the plurality ofdecoded signals A, B, C and D.

FIG. 8 is a schematic circuit diagram showing the auto-precharge timingcontrol unit 400.

Asshown, the auto-precharge timing control unit 400 includes a delayunit 410 and a signal output unit 420.

The delay unit 410 serves to delay the auto-precharge performing signalapcgpb in response to the decoded signals A, B, C and D, and serves tooutput the delayed auto-precharge performing signal apcgpb to the signaloutput unit 420. The signal output unit 420 receives the decoded signalsA, B, C and D, the auto-precharge performing signal apcgpb and thedelayed auto-precharge performing signal apcgpb from the delay unit 410.

If all of the decoded signals A, B, C and D are inactivated, the signaloutput unit 420 outputs the auto-precharge performing signal apcgpb. Onthe other hand, if one of the decoded signals A, B, C and D isactivated, the signal output unit 420 outputs the delayed auto-prechargeperforming signal apcgpb.

The delay unit 410 includes first to fourth unit delays 411 to 414,first to fourth transfer gate T1 to T4 and a plurality of inverters I27to I33.

The first unit delay 411 receives the auto-precharge performing signalapcgpb to delay it. The first transfer gate T1 is turned on by thedecoded signal A in order to transfer an output signal from the firstunit delay 411 to a latch formed by the 132 and the 133. The second unitdelay 412 delays an output signal from the first unit delay 411. Thesecond transfer gate T2 is turned on by the decoded signal B in order totransfer an output signal from the second unit delay 412 to the latch.

Likewise, the third unit delay 413 delays an output signal from thesecond unit delay 412. The third transfer gate T3 is turned on by thedecoded signal C in order to transfer an output signal from the thirdunit delay 413 to the latch. The fourth unit delay 414 delays an outputsignal from the third unit delay 413. The fourth transfer gate T4 isturned on by the decoded signal D in order to transfer an output signalfrom the fourth unit delay 414 to the latch.

The latch formed by the inverters I32 and I33 serves to store a signaloutputted from the plurality of decoded signals A, B, C and D.

The signal output unit 420 includes a NOR gate NOR1, a fifth transfergate T5, a sixth transfer gate T6 and two inverters I34 and I35. The NORgate NOR1 receives the plurality of decoded signals A, B, C and D toperform a logic NOR operation to the plurality of decoded signals A, B,C and D, and outputs the result to the I35.

If an output signal from the inverter I35 is in a logic low level, thesixth transfer gate is turned on and transfers the auto-prechargeperforming signal apcgpb as an output of the signal output unit 420. Onthe other hand, if the output signal from the inverter I35 is in a logichigh level, the fifth transfer gate T5 is turned on and transfers thedelayed auto-precharge performing signal apcgpb outputted from the delayunit 410 as the output of the signal output unit 420.

An operation of the semiconductor memory device in accordance with thepreferred embodiment of the present invention is described belowreferring to FIGS. 5 to 8.

The CAS latency mode detecting unit 100 selectively activates andoutputs the first and the second CAS latency control signals CL_S₁ andCL_S2 in response to the first to the fourth CAS latency mode signalsCL2 to CL5. For instance, if the CAS latency is 3, the CAS latency modedetecting unit 100 activates the first CAS latency control signal CL_SLand outputs the activated first CAS latency control signal CL_SL to theauto-precharge timing decoder 300.

Meanwhile, the timing control unit 11 generates the first and the secondprecharge control signals apcg1 and apcg2 by selectively blowing fusesincluded in the timing control unit 11.

Thereafter, the internal decoder 310 decodes the first and the secondprecharge control signals apcg1 and apcg2, and outputs the decodedsignals to the signal mixing unit 320. The signal mixing unit 320performs a logic operation to outputted signals from the internaldecoder 310 and the first and the second CAS latency control signalsCL_S₁ and CL_S2 in order to activate and output one of the plurality ofdecoded signals A, B, C and D.

Thereafter, the auto-precharge timing control unit 400 delays theauto-precharge performing signal apcgpb in response to the plurality ofdecoded signals A, B, C and D. An activated signal among the pluralityof decoded signals A, B, C and D determines delay amount of theauto-precharge performing signal apcgpb.

For instance, if the decoded signal C is activated, the third and thefifth transfer gates T3 and T5 are turned on and the other transfergates, i.e., T1, T2, T4 and T6, are turned off. Therefore, theauto-precharge performing signal apcgpb is delayed passing throughthefirst to the third unit delays 411 to 413 and outputted through thefifth transfer gate T5.

If all of the plurality of decoded signals A, B, C and D areinactivated, the sixth transfer gate T6 is turned on and the othertransferring gates, i.e., T1 to T5, are turned off. Therefore, theauto-precharge performing signal apcgpb is not delayed and outputtedfrom the auto-precharge timing control unit 400.

Thereafter, in response to an output signal of the auto-precharge timingcontrol unit 400, the auto-precharge operation is performed.

As described above, the semiconductor memory device in accordance withthe present invention can control timing of the auto-precharge operationaccording to a CAS latency. Therefore, the semiconductor memory devicecan perform the auto-precharge operation at proper time even if the CASlatency is changed.

Controlling timing of the auto-precharge operation according to the CASlatency means that the write recovery time is controlled according tothe CAS latency. That is, the semiconductor memory device in accordancewith the present invention can change the write recovery time when thewrite recovery time is required to be changed due to the change of theCAS latency.

A major cause of the change of the CAS latency is a change of a clockfrequency. Therefore, the semiconductor memory device in accordance withthe present invention can operate stably and reliably even if the clockfrequency and the CAS latency are changed.

The present application contains subject matter related to Korean patentapplication No. 2003-76796, filed in the Korean Patent Office on Oct.31, 2003, the entire contents of which being incorporated herein byreference.

While the present invention has been described with respect to theparticular embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A semiconductor memory device capable of controlling a timing of anauto-precharge operation, comprising: a CAS latency mode detecting meansfor outputting a CAS latency control signal in response to a CAS latencymode; and an auto-precharge control means for controlling the timing ofan auto-precharge operation in response to the CAS latency controlsignal.
 2. The semiconductor memory device as recited in claim 1,wherein the auto-precharge control means includes: a timing control unitfor generating a control signal to control timing of the auto-prechargeoperation; an auto-precharge timing decoder for activating one ofplurality of output signals from the auto-precharge timing decoder bydecoding the CAS latency control signal and the control signal; and anauto-precharge timing control unit for controlling output timing of anauto-precharge performing signal in response to an output signal fromthe auto-precharge timing decoder.
 3. The semiconductor memory device asrecited in claim 2, wherein the timing control unit includes a pluralityof fuses and outputs the control signal by selectively blowing theplurality of fuses.
 4. The semiconductor memory device as recited inclaim 2, wherein the CAS latency mode detecting means includes aplurality of unit CAS latency mode detectors, each for selectivelyactivating and outputting the CAS latency control signal according tothe CAS latency mode.
 5. The semiconductor memory device as recited inclaim 4, wherein each of the plurality of unit CAS latency modedetectors includes a logic gate for performing a logic operation to twoCAS latency modes for outputting the resultant of the logic operation.6. The semiconductor memory device as recited in claim 2, wherein theauto-precharge timing decoder includes: an internal decoder for decodingthe control signal; and a signal mixing unit for performing a logicoperation to a plurality of signals outputted from the internal decoderand the CAS latency mode control signal to thereby output the resultantof the logic operation.
 7. The semiconductor memory device as recited inclaim 6, wherein the auto-precharge timing control unit includes: adelay means including a plurality of unit delays for delaying theauto-precharge performing signal depending on the plurality of outputsignals from the auto-precharge timing decoder; and a signal output unitwhich receives the auto-precharge performing signal and an output signalfrom the delay means for outputting the auto-precharge performing signalwhen all of the plurality of output signals from the auto-prechargetiming decoder are inactivated and for outputting the output signal fromthe delay means when one of the plurality of output signals from theauto-precharge timing decoder is activated.
 8. The semiconductor memorydevice as recited in claim 7, wherein the delay means includes: a firstunit delay for delaying the auto-precharge performing signal; a firsttransferring gate turned on by a first output signal among outputtedsignals from the auto-precharge timing decoder for transferring anoutput signal from the first unit delay; a second unit delay fordelaying an output signal from the first unit delay; a secondtransferring gate turned on by a second output signal among outputtedsignals from the auto-precharge timing decoder for transferring anoutput signal from the second unit delay; a third unit delay fordelaying an output signal from the second unit delay; a thirdtransferring gate turned on by a third output signal among outputtedsignals from the auto-precharge timing decoder for transferring anoutput signal from the third unit delay; a fourth unit delay fordelaying an output signal from the third unit delay; a fourthtransferring gate turned on by a fourth output signal among outputtedsignals from the auto-precharge timing decoder for transferring anoutput signal from the fourth unit delay; and a latch for latching anoutput signal from the first to fourth transferring gates.
 9. Thesemiconductor memory device as recited in claim 7, the signal outputunit includes: a logic gate for performing a logic operation to theplurality of output signals from the auto-precharge timing decoder; afirst transferring gate which is turned on when an output signal fromthe logic gate is in a logic low level to output the auto-prechargeperforming signal; and a second transferring gate which is turned onwhen the output signal from the logic gate is in a logic high level tooutput the output signal from the delay means.
 10. A method ofcontrolling timing of a precharge operation according to a CAS latencymode comprising a step of: detecting the CAS latency mode; outputting adelay signal corresponding to the CAS latency mode; outputting anauto-precharge signal after delaying the auto-precharge signal bypassing the auto-precharge signal through one or more unit delays; andperforming an auto-precharge operation in response to the auto-prechargesignal, wherein the number of the unit delays where the auto-prechargesignal passes through is determined by the delay signal.